MEMBAR_OPS(3) | Library Functions Manual | MEMBAR_OPS(3)

# NAME

membar_ops, membar_enter, membar_exit, membar_producer, membar_consumer, membar_sync — memory access barrier operations

# SYNOPSIS

#include <sys/atomic.h>

void
membar_enter(void);

void
membar_exit(void);

void
membar_producer(void);

void
membar_consumer(void);

void
membar_datadep_consumer(void);

void
membar_sync(void);

# DESCRIPTION

The membar_ops family of functions provide memory access barrier operations necessary for synchronization in multiprocessor execution environments that have relaxed load and store order.

membar_enter()
  Any store preceding membar_enter() will reach global visibility before all loads and stores following it.

  membar_enter() is typically used in code that implements locking primitives to ensure that a lock protects its data.

membar_exit()
  All loads and stores preceding membar_exit() will reach global visibility before any store that follows it.

  membar_exit() is typically used in code that implements locking primitives to ensure that a lock protects its data.

membar_producer()
  All stores preceding the memory barrier will reach global visibility before any stores after the memory barrier reach global visibility.
membar_consumer()
  All loads preceding the memory barrier will complete before any loads after the memory barrier complete.
membar_datadep_consumer()
  Same as membar_consumer(), but limited to loads of addresses dependent on prior loads, or ‘data-dependent’ loads:

      int **pp, *p, v;

      p = *pp;
      membar_datadep_consumer();
      v = *p;
      consume(v);

  Does not guarantee ordering of loads in branches, or ‘control-dependent’ loads -- you must use membar_consumer() instead:

      int *ok, *p, v;

      if (*ok) {
      	membar_consumer();
      	v = *p;
      	consume(v);
      }

  Most CPUs do not reorder data-dependent loads (i.e., most CPUs guarantee that cached values are not stale in that case), so membar_datadep_consumer() is a no-op on those CPUs.

membar_sync()
  All loads and stores preceding the memory barrier will complete and reach global visibility before any loads and stores after the memory barrier complete and reach global visibility.

# SEE ALSO

atomic_ops(3)

# HISTORY

The membar_ops functions first appeared in NetBSD 5.0. The data-dependent load barrier, membar_datadep_consumer(), first appeared in NetBSD 7.0.

November 20, 2014 | NetBSD 7.2